Cerium-based abrasives (hereinafter simply called abrasives) have frequently been used to polish optical lenses conventionally. Recently, they have also widely been used as abrasives for glass materials used in electric and electronic apparatuses, including glass for magnetic recording media such as hard disks and glass substrates of liquid crystal displays (LCD's).
Cerium-based abrasives are manufactured from, for example, cerium-based rare earth carbonates (hereinafter also called rare earth carbonates) obtained from bastnasite ores or complex ores produced in China, or cerium-based rare earth oxides (hereinafter also called rare earth oxides) obtained by calcining rare earth carbonates at high temperature in advance as follows. First, these raw materials for cerium-based abrasives (hereinafter simply called raw materials) are wet pulverized by a pulverizing apparatus such as an attriter, a ball mill or a bead mill, and then are subjected to chemical treatment (wet treatment), followed by filtration and drying. After that, roast by heating appropriately sinters the particles of the raw materials to each other. The raw material after sintering is dry or wet disintegrated (re-pulverized) using such a pulverizing apparatus as described above and then the raw material after disintegration is classified. Such treatment provides an abrasive with desired particle diameters and a particle size distribution. In addition, the chemical treatment in this case refers to the treatment (treatment by mineral acids) of removing alkaline metals such as sodium, which give rise to abnormal particle growth during roasting, and to the treatment (fluorination) of adding a fluorine component for the purpose of attaining the polishing ability of a cerium-based abrasive and attaining the smoothness of a face to be polished. A fluorine component reacts with glass, i.e. a material to be polished, to enhance the smoothness of a face to be polished and the polishing ability. Therefore, fluorination can offer such effects.
Incidentally, abrasive products should not contain coarse particles. This is because coarse particles scratch a face to be polished. In addition, for example, a polishing step conducted in a manufacturing step for a glass substrate used for a magnetic recording medium capable of high density recording and high speed reading and writing requires a very high degree of precision in the smoothness and the like of the surface (face to be polished) of a glass substrate. Although these demands need to be satisfied, a high concentration of coarse particles in an abrasive is liable to generate scratches on the surface of a glass substrate; as a result, requirements such as smoothness cannot be attained. Accordingly, in this respect, it is desirable that an abrasive should not contain coarse particles.
Furthermore, taking into account polishing operation efficiency in a polishing step, an abrasive product needs a high polishing ability. Thus, in order to attain a high polishing ability, an abrasive needs to be pulverized so that the particle diameter is not lowered more than necessary.
Incidentally, conventional pulverizing means has some limit, so that it cannot satisfy these requirements. That is, reduction of the number of coarse particles by conventional wet pulverization using pulverizing apparatuses such as a ball mill, an attriter and a bead mill requires elongated pulverizing duration, which in turn increases the amount of fine particles pulverized more than necessary, leading to the difficulty of attaining a polishing ability of the abrasive product.
Hence, conventional abrasives achieve the smoothness required for a face to be polished and polishing ability during polishing by adding a fluorine component by fluorination during abrasive production to utilize the effect of the fluorine component added. As described above, this is due to the fact that a fluorine component has the effects of enhancing the smoothness of a face to be polished and of enhancing polishing ability. For example, a method is disclosed in Japanese Patent Laid-Open No. 9-183966, in which an abrasive is manufactured by dropping hydrofluoric acid aqueous solution, with agitation, into raw material slurry after wet pulverization so as to make the content of fluorine in an abrasive product become 3% by weight to 9% by weight.
However, attainment of required smoothness and polishing ability by the addition of fluorine increases the concentration of the fluorine component in the abrasive, thereby easily adhering the fine particles of the abrasive to a face to be polished during polishing and also easily causing the particles to remain on the face polished, leading to the disadvantage of lowering cleanability for the face polished.
The present invention has been made in the background described above and offers a subject to provide a method of manufacturing a cerium-based abrasive having coarse particles in lower concentration and having higher polishing ability and also having excellent cleanability for a polished face.